In cellular mobile communication systems, each basestation is assigned a physical layer identifier, allowing user equipment devices (UEs) to distinguish between signals transmitted from different basestations. Physical layer identifiers may be realized in different ways according to different mobile communication standards. Two examples of physical layer identifiers are scrambling codes in WCDMA (Wideband Code Division Multiple Access) and Physical Layer Cell Identifiers (PCIs) in LTE (Long Term Evolution).
Physical layer identifiers are assigned to basestations in such a manner as to ensure that two operating conditions may be fulfilled. According to a first condition, known as collision avoidance, it should be ensured that a single UE does not simultaneously receive the same physical layer identifier (e.g. scrambling code or PCI) from more than one basestation or cell. Failure to meet this condition can have an adverse impact on call drop rate owing to UE failure to report neighboring basestations on an overlapping identifier. Call termination and call establishment success rate can also be affected as a result of UEs failing to detect a change in cell, and so failing to decode the relevant system information associated with the new cell.
According to another condition for physical layer identifiers, known as confusion avoidance, neighbor ambiguity should be avoided when a UE reports measurements for a particular physical layer identifier. Even if collision avoidance is satisfied, physical layer identifiers may be reused in geographically separate or distant locations. It is important for the network to be able to identify the correct cell in the context of its local neighbors for correct handover targeting. Failure to meet this condition can have an adverse impact on handover success rates and ultimately on call drop rates.
The established approach to ensuring compliance with the above discussed conditions is to use network cell planning techniques, followed by auditing and optimization of the system. During a network planning process, network planning tools automatically assign physical layer identifiers based on predicted cell adjacencies determined using radio propagation models. These identifier assignments are later transferred to the live network through a network management system. This established approach is effective for coordinated, pre-planned network deployment. However, the rapid growth in scale and complexity of mobile communications networks mean that this fully planed approach is becoming a large and difficult overhead for network operators to maintain. The drive to increase system capacity (the number of users supported by the system) and improve end user experience (for example through increased rates of data transfer) is leading to significant increases in the density of basestations being deployed. With so many basestations in the network, a traditional planning phase is no longer feasible before each deployment, and “ad-hoc” basestation deployment is becoming increasingly necessary. In ad-hoc deployments where traditional planning is not available, the basestation itself automatically performs the auditing and optimization steps of a physical layer identifier selection.